<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button> 点我</button>
    <script>
        let button = document.querySelector('button')
        button.addEventListener("click", () => alert("1"));
        button.removeEventListener("click", () => alert("1"));
        button.onclick = () => alert(2);
          //  先后显示1，2
          //  都会运行
    </script>
</body>

</html>